The specific aims of this protocol is to establish whether the addition of an anti-resorptive drug enhances parathyroid hormone's ability to increase bone mass (whether PTH and an anti-resorptive drug have additive or synergistic effects on bone mass), in male and postmenopausal female patients with severely reduced bone mass. If so, the combination will be potent enough to reverse osteoporosis in many women and/or men. The second specific aim of this protocol is to establish the extent to which parathyroid hormone'' ability to stimulate bone formation requires concomitant bone resorption, in male and postmenopausal female patients with severely reduced bone mass.